Jill and Leon are basically tied in popularity. Have been for a long time. Now did Leon get a boost from Resident Evil 2 Remake? Definitely, so yeah maybe he's ahead right now but their popularity has always been nearly identical.
Personally, I'm a Leon boy. Resident Evil 4 wasn't just my first RE it was my very first M Rated game. Fell in love with the character in that game and even more so as I played retroactively through 2, 6, and Darkside Chronicles.
The issue lies in the fact of which one do you go with? On one hand, at the moment, Leon is more profitable and is a part of 4, the most recognized game of the franchise, 6, the best selling game in the franchise, and 2 and 2 Remake, which are fan favorites to say the VERY least, OR do you go with Jill? One of the characters that KICKED OFF the franchise, has appeared in multiple Capcom crossovers, and is overall seen as one of THE MOST recognized and popular female protagonists in gaming.
It's honestly a coin toss which is why I'm hoping they either go with an alternate costume take featuring Chris, Jill, Leon, Claire, etc. Or just bite the bullet and make this a special case where a DLC character gets an Echo with slight differences. Honestly, I think a RE character is inevitable but if I HAD to choose one over the other it'd absolutely be Leon. Simply because he has YET to be a BIG crossover character of Capcom's and I feel like he deserves it.